1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines for controlling the air-fuel ratio of a mixture supplied to the engine in response to outputs from oxygen sensors arranged in the exhaust system of the engine, respectively, upstream and downstream of a catalytic converter arranged in the exhaust system.
2. Prior Art
Conventionally, an air-fuel ratio control system is known, which includes oxygen sensors arranged in the exhaust system of an internal combustion engine, respectively, upstream and downstream of a catalytic converter (e.g. three-way catalyst) arranged in the exhaust system, and controls the air-fuel ratio of a mixture supplied to the engine in a feedback manner responsive to outputs from these oxygen sensors, so as to improve exhaust emission characteristics of the engine. According to this air-fuel ratio control system, an air-fuel ratio correction coefficient KO2 which has a value thereof determined by the output from the oxygen sensor upstream of the catalytic converter (hereinafter referred to as "the upstream oxygen sensor"), or a reference value for determining whether the air-fuel ratio is lean or rich, which is compared with the air-fuel ratio correction coefficient KO2 is changed by a feedback control constant based on the output from the oxygen sensor downstream of the catalytic converter (hereinafter referred to as "the downstream oxygen sensor"), to thereby compensate for deterioration of the upstream oxygen sensor, etc.
The above feedback control based on the output from the upstream oxygen sensor undergoes a problem that when the temperature of a catalyst of the catalytic converter (hereinafter simply referred as "the catalyst temperature") is low, the maximum oxygen storage capacity of the catalytic converter is low and unstable and accordingly the output from the downstream oxygen sensor is unstable, which can result in hunting of the value of the air-fuel ratio correction coefficient, etc. To solve this problem, it has been proposed to detect the temperature of the engine, the catalyst temperature or the like, and interrupt the feedback control based on the output from the downstream oxygen sensor when the detected temperature is lower than a predetermined value, e.g.. by Japanese Laid-Open Patent Publications Nos. 61-237858 and 63-97848).
Further, also when the catalytic converter is deteriorated, the oxygen storage capacity of the catalytic converter lowers. In view of this fact, it has also been proposed to interrupt the feedback control based on the output from the downstream oxygen sensor when the catalytic converter is deteriorated, e.g. by Japanese Laid-Open Publication No. 63-205441.
However, none of the above proposed methods make it possible to obtain results of the feedback control based on the output from the downstream oxygen sensor, i.e. effects such as prevention of degradation of exhaust emission characteristics of the engine ascribable to deterioration of the upstream oxygen sensor, etc., until after the catalytic converter has risen in temperature enough to become fully activated. This will be explained in detail with reference to FIG. 15 showing the maximum oxygen storage amount relative to the catalyst temperature. As shown in the figure, according to the conventional proposed methods, when the catalyst temperature TCAT is lower than a predetermined value (e.g. 400.degree. C.), it is presumed that the catalytic converter is not activated, and then the feedback control based on the output from the downstream oxygen sensor is inhibited. However, as is understood from the figure, even when the catalyst temperature TCAT is below the predetermined value, if the catalytic converter is in a half-activated state, i.e. incompletely activated state (e.g. 200.degree.-400.degree. C.), it has some or less oxygen storage capacity. Nevertheless, according to the conventional proposed methods, even when the catalytic converter is in such a half-activated state, the feedback control based on the output from the downstream oxygen sensor is inhibited, thus failing to reduce emissions of noxious exhaust gas components on such an occasion.
Moreover, a feedback control constant, which is e.g. a proportional term, is employed to correct the air-fuel ratio correction coefficient KO2. If the feedback control constant is updated with an updating rate set with the catalytic converter being in an activated state, when the catalyst temperature is so low that the catalytic converter is not fully activated, the change rate of the feedback control constant becomes large due to a small maximum oxygen storage amount of the catalytic converter in a half-activated state. As a result, the change rate of the air-fuel ratio correction coefficient KO2 increases so that the air-fuel ratio of exhaust gases downstream of the catalytic converter fluctuates, rather leading to exhaust emission characteristics downstream of the catalytic converter.